The PIN photodiode is an element that has a P-I-N structure with an intrinsic layer (high resistance epitaxial layer or the like) between a p-type semiconductor and an n-type semiconductor, and that converts incident light to a photocurrent. Its principle of operation is as follows: when light with energy higher than the band-gap energy is incident on silicon (Si) having a PIN structure with a reverse bias applied to it, electron-hole pairs are generated in the silicon crystal. As photocarriers, electrons move to the N layer, while holes move to the P layer, and a back current is output.
For example, Patent Reference 1 has disclosed a method of manufacturing a photodiode characterized by the following facts: as shown in FIG. 6(a), p-type semiconductor layer 12 is formed on the outer layer of n-type semiconductor layer 11; mask layer 30 and insulating layer I are formed on p-type semiconductor layer 12; and, as shown in FIG. 6(b), with mask layer 30 being used as an etching stopper, opening H is formed in insulating layer I; and mask layer 30 within opening H is removed by means of wet etching to form the photodiode. As a result, it is possible to inhibit leakage caused by etching damage.
In addition, Patent Reference 1 has disclosed a type of photodiode characterized by the following facts: as shown in FIG. 7, plural p-type semiconductor layers 12 are formed in a checkerboard configuration in n-type semiconductor layer 11, and antireflection film AR made of silicon oxide film 25 and silicon nitride film 26 is formed on the silicon surface to create the photodiode.    [Patent Reference 1] Japanese Kokai Patent Application No. 2001-20079
Due to its properties, silicon can only convert light with wavelengths in the range of 400-1100 nm into a photocurrent for output. This is because the photon energy of light with a wavelength longer than 1100 nm is lower than the band-gap energy (1.12 eV) of silicon, so that the electron-hole pair cannot be formed. On the other hand, the short-wavelength light can generate the electron-hole pair only near the silicon surface. Because the silicon surface usually has a very high recombination speed, the electron-hole pairs formed under light with a wavelength shorter than 400 nm are immediately recombined, and the photocarriers in the silicon are annihilated.
The PIN photodiode has two important characteristics, namely sensitivity (photoelectric conversion efficiency) and BW (response speed). FIG. 8 is a cross section illustrating a PIN photodiode with a constitution formed taking into consideration said two important characteristics with respect to the blue light wavelength (λ=405 nm) based on the fundamental principle of the PIN photodiode. As shown in the figure, low concentration p-type silicon layer 41 is formed on high-concentration p-type monocrystalline silicon substrate 40, and low concentration n-type silicon layer 42 is formed by epitaxial growth on said p-type silicon layer. When a reverse bias voltage is applied, a depletion region is formed between said p-type silicon layer 41 and n-type silicon layer 42, and when light is incident on it, electron-hole pairs are generated.
Intermediate concentration n-type silicon regions 43, 44 are formed in n-type silicon layer 42. Plural silicon regions 43 are arranged in a grid configuration on the light receiving plane. High concentration n-type contact regions are formed in said silicon regions 43, 44, respectively, and contact regions are respectively connected to the platinum silicide (PtSi) or other electrodes 45, 46.
High concentration p-type channel stop region 48 connected to p-type silicon layer 41 is formed below field oxide film 47. Metal wiring 49 is electrically connected via high concentration p-type contact region 50 to p-type silicon layer 41, and metal wiring 51 is connected to electrode 46. Electrode 45 within the light receiving plane is electrically connected to metal wiring 51 at a position not shown in the figure. Multilayer insulating layer 52 is formed on field oxide film 47. Opening H is formed in multilayer insulating layer 52 to define the light receiving plane. The silicon surface exposed in opening H is covered with silicon nitride film 53, and its upper surface is covered with silicon nitride protective film 54.
With a reverse bias voltage applied between metal wirings 49, 51, a depletion region is formed between p-type silicon layer 41 and n-type silicon layer 42. Because n-type silicon layer 42 is much thinner than p-type silicon layer 41, the depletion region reaches the silicon surface. Electron-hole pairs are generated in the depletion region when light is incident on it. The holes flow from p-type silicon layer 41 to metal wiring 49, electrons flow to electrode 45 near the depletion region, and the current obtained by photoelectric conversion is output.
Grid-like high concentration silicon regions 43 and electrodes 45 are placed near the surface in the PIN photodiode so that the short wavelength blue light can generate photocarriers near the surface of silicon. As a result, a low concentration layer is formed between the grids, the depletion layer can spread effectively, and photocarriers can be generated even at the blue light wavelength. Because said grid-like high concentration silicon regions 43 and electrodes 45 are arranged adjacent to the depletion region where the photocarriers are generated, the generated photocarriers can move smoothly through high concentration silicon regions 43 toward electrodes 45 before being annihilated in silicon to form a photocurrent, and the photoelectric conversion efficiency is optimized with respect to the blue light wavelength. Here, the photoelectric conversion efficiency refers to the ratio of the current obtained by photoelectric conversion to the power of the incident light.
The photoelectric conversion efficiency is as high as about 0.284 A/W for a PIN photodiode using the blue light wavelength with this constitution. However, the theoretical threshold is 0.327 A/W, which has not yet been reached. In order to achieve a value nearer to the theoretical threshold, electrodes 45 should not be positioned within the light receiving plane (opening H). Said electrodes 45 within the light receiving plane block the incident light, so that the number of carriers generated in the depletion region decreases and the photocurrent therefore falls, contributing significantly to a decrease in the photoelectric conversion efficiency. On the other hand, if there are no electrodes 45 in the light receiving plane, the travel distance of the carriers increases and the proportion of carriers annihilated due to recombination of the carriers near the silicon surface becomes higher, leading to a decrease in the photoelectric conversion efficiency. This is an antinomy topic.
The purpose of the present invention is to solve the aforementioned problems of the prior art by providing a type of photodiode and a method of manufacturing the photodiode having a higher photoelectric conversion efficiency (sensitivity) than that in the prior art.
In addition, a purpose of the present invention is to provide a type of photodiode and a method of manufacturing the photodiode with a high photoelectric conversion efficiency for blue light or other short wavelength light.